Electrodeless light sources which operate by coupling high frequency power to a high pressure arc discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor surrounding the inner conductor as described in U.S. Pat. No. 3,942,058 issued Mar. 2, 1976 to Haugsjaa et al. and U.S. Pat. No. 3,942,068 issued Mar. 2, 1976 to Haugsjaa et al. The electrodeless lamp is positioned at the end of the inner conductor and acts as a termination load for the fixture. The termination fixture has the function of matching the impedance of the electrodeless lamp during high pressure discharge to the output impedance of the high frequency power source. Thus, when the high pressure discharge reaches steady state, a high percentage of input high frequency power is absorbed by the discharge in the electrodeless lamp.
Previous patents describe electrodeless light sources wherein the termination fixture couples power to one end of the electrodeless lamp. While light sources with single-ended coupling give generally satisfactory results, they have certain disadvantages. In the situation where power is coupled to one end of the lamp and the other end is open-circuited, the electric field in the lamp decreases with increasing distance from the power coupling conductor. As a result, arc intensity also decreases with increasing distance from the power coupling conductor.
Non-uniform arcs are undesirable for several reasons. They produce both hotspots and coldspots in the wall of the lamp envelope. Hotspots occur adjacent to points of maximum arc intensity and at points where the arc attaches to the lamp envelope. The envelope wall material has a maximum operating temperature. Therefore, the total power which can be delivered to the lamp without exceeding the maximum temperature is reduced by the existence of hotspots. The light output of the lamp is correspondingly lowered. Moreover, for a given value of input power, the life of the lamp is reduced when hotspots occur. Coldspots occur at the points on the lamp wall which are most distant from the arc and are undesirable because fill material can condense on the lamp envelope at coldspots and can block a portion of the light output by absorption. Conversely, a more uniform arc results in a more uniform wall temperature and a higher level of input power and light output can be achieved. Also, the life of the lamp is increased when temperature variations over the wall of the lamp are minimized.
It is frequently desirable to use elongated light sources. For example, elongated fluorescent lamps are commonly used in homes and offices. Also, elongated light sources are used in various scientific applications such as in laser pumping. In the case of electrodeless lamps with single ended power coupling, the intensity of the arc decreases as a function of distance from the power coupling conductor. Electrodeless lamps of more than a few centimeters in length are, for this reason, impractical. The arc can be extended by increasing the input power. However, the problems of high lamp wall temperatures and of attachment of the arc to the lamp wall place limitations on input power increases. Longer electrodeless lamps could more easily be achieved if the arc intensity was uniform.
Another problem with single ended coupling relates to the orientation of the lamp during discharge. The optimum orientation for single ended coupling is with the lamp in a vertical position and with power coupled from the bottom. In this position, heat generated by the arc is carried upwards in the lamp by convection currents which have the additional effect of extending the arc upwards, thereby increasing its length. This effect is reversed if power is coupled to the lamp from its top. Convection currents again carry heat upwards in the lamp, but the effect is to shorten the arc which extends downward from the power coupling conductor. Convection currents have an effect on the arc whatever the orientation of the lamp. Thus, the performance of lamps with single ended coupling varies with orientation. Since light sources are normally required to operate in a variety of orientations, it would be desirable to construct an electrodeless light source wherein the susceptibility to changes in orientation is reduced.